Devices providing adjustable optical power have been proposed for use as corrective lenses in prescription eyeglasses. In U.S. Pat. No. 3,305,294 by Alvarez an approach is described where two separate, specially shaped optical plates are moved relative to each other along an axis perpendicular to a central viewing axis along which the optical elements are arranged. The variation of thickness of each of the plates is described by a cubic polynomial of Cartesian (x,y) coordinates in a plane perpendicular to the central viewing axis. The variation in thickness is complementary in that when the plates are placed in a neutral position (defined as where the origin of the cubic thickness profiles of each of the plates lies on the optical axis) the cubic polynomial terms for the two plates have the same magnitude as each other but opposite sign. This complementary design of thickness variations results in a device that provides a variable amount of optical power when the plates are aligned together and slid relative to each other along the designated axis and viewed along a suitably arranged axis. Others have continued the work by Alvarez. In one example, WO 2013/030603 describes changes to the surface profile away from a strict cubic polynomial form. These changes add terms and/or functions to the cubic polynomial with the aim of providing better off-axis viewing. In general, a lens formed of lens plates having surface profiles or thicknesses as described by Alvarez or derived from the form described by Alvarez may be known as an “Alvarez-type lens”.
One of the problems of the Alvarez type lens is that they consist of a pair of lenses which are held close to each other or are in contact with each other. Like all corrective eyewear the lenses are exposed to dirt and moisture from the surrounding environment and sweat and grease from the user's body. Hence, the ability to maintain clean lenses is of particular importance. For the Alvarez type lenses it is even more important to be able to keep the lenses clean because there are double the number of optical surfaces compared to conventional fixed magnification corrective eyewear.
A number of approaches have been developed in attempt to overcome the problems of maintaining the Alvarez lens pair clean. In WO 2011/086177 corrective eyeglasses are described in which a hinge mechanism links the optical plates of the Alvarez-type lens. The hinge allows one lens plate to be swung away to permit easy cleaning of both plates of the Alvarez-type lens. The hinge is incorporated along the length of the top of the lens. To provide aesthetically attractive eyeglasses it then becomes desirable to conceal the hinge which results in an increase in the size of the frames of the eyeglasses.
An alternative approach is to conceal the periphery of the optical plates within the eyeglasses frame. Here the edges of the optical plates remain concealed in the eyeglasses frame as the optical power is adjusted and the plates move. The frames are required to be bulky to accommodate this movement. Alvarez-type lenses can be arranged such that one plate remains fixed while the other plate is moved, or where both lens plates are translated. In both cases under continued use the optical plates remain prone to becoming covered with dirt and moisture between the plates. The integration of the plates into the frame means that it is difficult clean between them without completely dismantling the eyeglasses.
In GB 2486212 there is described a self-contained lens unit incorporating the Alvarez lens plates. The lens unit is formed as a capsule. The capsule is formed of one of the Alvarez plates and a cover which forms the front and rear surfaces of the capsule with a cavity there between. The Alvarez plate and cover are sealed together by a peripheral wall. The second Alvarez lens plate resides in the cavity between the first plate and cover. The second plate is arranged for translation in the cavity, which is actuated by a rod passing through the peripheral wall of the cavity and coupling with the second plate. An elastomeric seal is used around the rod at the peripheral wall of the capsule to prevent dirt and moisture ingress into the cavity. The rod has a thread which engages with a thread in the peripheral wall of the capsule. A problem with this threaded drive is that to achieve a convenient amount of power change per turn of the rod requires a thread with multiple starts. This can be difficult to manufacture, increasing cost and complexity.